Thanks JBarca, I think you nailed it. Starting with a cold hot water tank, I turned on the heater and did not flow any water. It started dripping 10 to 20 minutes later. I flowed some water, and it stopped.
The water was dripping from the bottom of the pump. Both inlet and outlet connections felt dry. Since this is a brand new trailer, I’m not sure whether I should go back to the dealer, or put in some kind of accumulator myself.
At least I have a work-around. When I turn on the heater, wait 10 minutes and flow some water. Then don’t turn off the heater until I really need to.